Method of separating gas mixtures



Sept. 1, 1953 w. E. LOBO METHOD oF SEPARATING @As MIxTuREs Nm, `N

INVENmR.

ATTONEYs N EULUI I WALTER 'BY 4 r j# @0M Patented Sept. y1, 1953 METHODF SEPARATING GAS MIXTURES Walter E. Lobo, Westfield, N. J., assigner toThe M. W. Kellogg Company, Jersey City, N. J., a corporation of DelawareApplication April 29, 1943, Serial No. 23,933

(Cl. (i2-175.5)

Claims.

' This invention relates to improvements in the separation of normallygaseous mixtures, containing higher boiling components as undesirableimpurities, by low temperature liquefaction and rectification. Morespecifically, the invention relates tothe removal of a higher boilingcomponent from a liquefied gas, such as one comprising carbon dioxide,air, oxygen, or hydrocarbons, the concentration being such that as arule the component does not materially change the boiling point of theliquefied gas, but is otherwise objectionable because it either impairsthe purityof the product or by its presence creates an explosive hazardin operating the separation process.

A specic object of this invention is to accomplish the substantiallycomplete removal of an impurity from a liquefied gas in a rectificationzone or to reduce the concentration thereof to a concentration Withinsafe operational limits.

Another object is to remove dissolved impurity from a liquefied gas bythe continuous and forced recirculation of the liquefied gas through abody of solid adsorbent material.

' A further object is to remove dissolved normally gaseous impurity froma liquefied gas by the ,continuous and forced recirculation of portionsof the liqueed gas through a body of granular particles of solidadsorbent material.

' A still further object is to remove dissolved normally gaseousimpurity from a liquefied gas by recirculating continuously through abody of solid adsorbent material a stream of the liqueiied gasmaintained in positive flow conditions by a7thermal siphon effectestablished and maintained by `partial vaporization of the circulatingliquefied gas.

It is also an object to utilize the heat derived from condensing vaporsof a higher pressure rectification zone to establish and maintain athermal siphon for the forced recirculation af a stream of a liquefiedproduct of a lower pressure rectiiication zone through a closed circuitincluding a body of adsorbent material for adsorbinga dissolved impurityfrom the liquefied product.

' Qther objects will become apparent in the more detailed explanation ofthe invention with reference to illustrative examples concerning theremoval of acetylene from the air in connection with the separation ofair by the Well-known procedure of low temperature rectification.

The separation of atmospheric air into oxygen-rich `and nitrogen-richproducts by liquefaction and rectification at low subatmospherictemperatures requires the use of relatively large quantities of air.Consequently, even insignicant amounts of impurities in the feed airreadily can become greatly magnified if not removed continuously. Duringthe forecooling Step of the process some higher boiling gaseousimpurities condense as liquids and solids on the cooling surfaces, andin time accumulate in such quantities as to restrict the gas flows andupset the plant operation. By employing the principle of reversing heatexchange to perform the main cooling step of the process, either bymeans of regenerators or exchangers, Water and carbon dioxide inparticular may be so effectively removed that long time plant operatingschedules are possible. Hydrocarbons, however, are seldom present in thefeed air in concentrations great enough to cause condensation andthereby their removal in the main forecooling step. Yet thesehydrocarbons must be removed from the plant by some means, as they notonly interfere with the flows but also introduce a hazard. By the natureof the process, traces of hydrocarbons in the feed air finallyaccumulate in the reservoir of oxygenrich liquid at the bottom of themain rectification stage. With a sufficient concentration ofhydrocarbons in the oxygen-rich liquid, the plant is liable to damagingexplosions in the event the reaction is detonated by some priming agent.It is known that acetylene is present in atmospheric air and that itaccumulates in the oxygen-rich liquid as a solid explo-sively sensitiveto shock, electro static charges, or even sudden temperature changes,particularly if ozone also is present in the liquid. While the acetyleneconcentrations in air are usually small, the saturation concentrationsin the oxygen-rich liquid are also small. For example, solid acetylenemay be precipitated when it reaches the low concentration of three partsper million by volume in the oxygen-rich liquid.

Beds of adsorptive materials have been employed at the cold outlet ofthe forecooling step, on the inlet line to the expander, and on theoxygen-enriched liquid air line of a conventional two stage rectifyingcolumn, these beds being designed to take advantage of the highadsorptivity of solid adsorbents Iat low temperatures and underpressure. The beds usually are relatively large `in size, require largeamounts of refrigeration for cooling preliminary to being placed inservice initially and after each regeneration period, and suffer theadditional disadvantage in that they are subject to by-passing eitherthrough faulty operation or through defects in construction. Thesedevices situated as they are in the plant processing arrangement, arenot capable of being used to remove any impurities that may haveby-passed the adsorbent in a period of faulty operation.

Hydrocarbons also have been removed from the oxygen-rich liquid by anevaporative procedure. This procedure, however, concentrates theexplosive impurities in a small stream of liquid oxygen that has to beWasted at the expense of 3 the plants refrigeration and, therefore,lowers the plants power economy. This evaporative method is hazardousbecause there is no certainty that all of the hydrocarbons are containedin the waste liquid stream since some, for example,

may be entrained in the rising vapors of evapora'-` tion and depositedin a location where they are particularly liable to explode. Also, greatcare must be exercised to avoid `explosions in the Vdisposal line.

The present invention will be illustrated by its application to theremoval of acetylene from the so-called low pressure rectiiica'tionstage of an air separation plant producing an oxygen-rich product.According to this application of the invention, the oxygen-rich liquidat the base of the low pressure stage is recirculated over adsorptivesolids which remove the acetylene at such a rate that, over theoperating cycle of the adsorber, precipitation of the dangerous solidphase of acetylene at all points of the circuit is avoided'. Thisprocedure eliminates acetylene at its point of final accumulation in theair separation plant. It accomplishes this removal in a safe procedurewithout allowing the acetylene to reach the dangerous or solid state inthe plant or inthe remaining equipment. Because of the recirculationprocedure, imperfections in the adsorption step are less serious sincetraces of acetylene that escape adsorption on their first pass throughthe adsorptive bed are eventually adsorbed in subsequent passes throughthe bed. Acetylene escaping adsorption during start-up periods isadsorbed later when the adsorptive bed is funetioning effectively.

For a more complete understanding of the process of the invention,reference is had to the following detailed description taken inconnection with the accompanying drawings, in which:

Figure l diagrammatically represents a portion of a two-stagerectification tower and a closed recirculation system for passingliquefied gas through a body of adsorbent in accordance with a preferredmodification.

Figure 2 diagrammatically shows an alternative arrangement of apparatusfor recirculating liquefied gas through a body of adsorbent material.

Figure 3 and 4 diagrammatically illustrate a further alternativearrangement for recirculating continuously through a body of adsorbentmaterial a stream of liquefied gas maintained in positive flowconditions.

Figures 5 and 6 show diagrammatically the application of the process ofthe invention according to the preferred modification in connection witha single stage rectification tower.

In the preferred modification, as applied to a two-stage rectificationoperation, the method of the invention utilizes a closed recirculationsystem wherein the liqueiied oxygen-rich liquid accumulating in thebottom of the low pressure, or main, rectication stage ispassed'continuously through a body of adsorbent material. A portion ofthis liquid is reboiled to provide vaporized product and the vaporupiiow for rectification. Circulation of liquid through the adsorbentpreferably is induced by a thermal flow effect established by the Siphonaction set up by a reboiler which may be within or external to therectifying column and may provide either part or all of the reboilingrequired. Heat for reboiling is obtained by heat exchange betweenvaporizing oxygen-rich liquid and. condensing vapors of nitrogen fromthe top of the preliminary, or high pressure, rectification stage.

According to one processing arrangement incorporating the preferredmodification wherein recirculation is effected. byA an` externalreboiler, the i'eboiling step is performed downstream from theadsorption step and in this event the vaporized and unvaporized portionsof acetylene-denuded liquid preferably are returned through a commonconduit to the vapor space below the bottom tray of the mainrectification stage. After vaporliquidseparation the unvaporized portionfrom the external reboiler is commingled with the main body ofoxygen-rich liquid in the rectifying tower. The vaporized portions arepartially withdrawn as product material and the remainder is permittedto pass upwardly/through the rectification stage. According to analternative processing arrangement, the reboiling step to establishliquid circulation is performed prior to the adsorption step in whichcase an internal reboiler preferably promotes positive fluid circulationandl the partial vaporizat'ion of the oxygen-rich liquid in the externalreboiler is carried out primarily to produce only the residualvaporization required. The primary requisite of the method is toradjustthe rate of flow of liquid through the adsorption step to keep theacetylene concentration in the liquid returning to the rectificationstage sufciently low so that when this liquid again is commingled withthe main body ofoXygen-rich liduid the overall acetylene concentrationtherein is maintained below the concentration at which crystalsacetylene precipitate for the calculated amount of vaporization in therectification column. This rate of flow therefore isa Variable conditiondependent upon the eiiiciency of' acetylene adsorption per pass inconjunction with the acetylene content inthe liquid downfiow of the mainrectification stage.

Referring now to Figure 1, rectification tower l which is constructed inthe conventional manner for rectifying air, is separated into twocompartments or stages denoted by the numerals 2 and 3'. For the purposeof this description only the top portion of stage 2 and the bottomportion of stage 3 are shown, as these portions only are necessary tothe description. Stage 2 represents the first, or high pressure,rectification stage wherein the precooled compressed air is rectifiedprimarily into liquefied oxygen-enriched air by removing overhead a partof its nitrogen content. The nitrogen thus separated is condensed andboth products of the primary rectification, preferably also subcooled,are then expanded into the second, or main, rectification stage 3,together with another portion of cold expanded air, for separation intonitrogen-rich or oxygen-rich iinal products. This latter rectificationis performed under a lower pressure than the pressure of the rstrectiiication stage.

The liquid downiiow of the second rectification accumulates in a pool atthe bottom of stage 3 around the outside of the tubes of the reboilerapparatus 4. Although the apparatus for reboiler 4 is shown in thefigure as being a capped bundle of tubes located in the bottom of stage3 but having the inside of each tube opening into the vapor space at thetop of stage 2,` the design of this reboile'r is notto be limited tothis specific arrangement. For example, a reboler designed somewhatsimilar to reboiler 50 of Figure 2 having suitable liquid and vaporconnecting lines to stages 2 and 3 would be just as satisfactory forcarrying out the function of reboiler 4. In any event theliquid at thebottom of rectiiication stage 3 is reboiled to provide the vapors forrectification and vaporous product to be withdrawn from the system. Toprevent build-up of liquid around the reboiler the liquid revaporizednecessarily must be equivalent to the liquid passing down therectication column into the pool. Since acetylene has a negligible vaporpressure under the conditions at which this vaporization takes place,only a negligible quantity of this material which enters the pool isremoved in the evolved vapors. The highest acetylene concentration inthe air separation system exists at this point since vaporization,particularly when producing a vaporous product, is the primary means forremoving material from the pool.

Figure l illustrates the removal of acetylene according to the inventionin an air separation process in which approximately 30,750 pounds perhour of oxygen-rich liquid enters the liquid pool surrounding reboiler 4from the bottom tray of rectification stage 3 of which 11,165 pounds perhour is subsequently removed through line 46 as vaporous product. Stage3 of the rectification operates under a pressure of pounds per squareinch absolute and this necessitates a reboiling temperature in theliquid pool of -290 F. The heat required to boil vapors from the pool,equivalent to the quantity of liquid entering it, is derived from thecondensation of 21,449 pounds per hour of nitrogen vapors reaching thetop of the high pressure rectification stage 2. This later stageoperates under a pressure of 105 pounds per square inch absolute so thatthe nitrogen vapors entering into heat exchange relation with theoxygen-rich liquid are approximately at -280 F. Inasmuch as theatmospheric air charged to the separation process may contain on theaverage about one part of acetylene per million parts of air on agaseous volume basis, hereinafter referred to simply as a certain numberof parts per million of acetylene, the downflowing oxygen-rich liquidcontinuously is bringing into the liquid pool around reboiler 4, abouttwo parts per million of acetylene. This acetylene must be removed withenough efficiency to keep the concentration of acetylene in the liquidpool around reboiler 4 below three parts per million so that therectification can be carried Vout within .safe operational limits.

The oxygen-rich liquid is circulated continuously through a body ofadsorbent material to remove its acetylene constituent. In the presentillustrative process, a maximum of 55% of the liquid surrounding thetubes of reboiler 4 is vaporized thereat so that the acetyleneconcentration in the remaining liquid cannot do more than double itsvalue. The process must be operated, however, in a manner to precludeprecipitation of solid particles of acetylene even in the event that theacetylene concentration in the liquid downow is doubled. 'I'his is doneby continuous recirculation of the oxygen-rich liquid through anadsorbent which eliminates any disadvantage arising from loweredadsorption eiciency, a deficiency attendant with once through procedure,and thus assures greater certaintyof removing enough acetylene tomaintain the required low concentration thereof in the reboiler liquid.

The adsorptive capability of the flow arrangement of Figure 1,therefore, is adaptable to variations in the concentration of acetylenein the atmospheric air being charged to the separation system. Thepresent process for describing Figure 1 is based upon an exemplary feedair mixture initially containing an average of acetylene 6 concentrationof one part per million. With a feed of this character and with a designin which the adsorption capacity of a bed of adsorbent is of suchproportions as to be capable of effectively removing substantially allof the acetylene from the oxygen-rich liquid contacting it in a singlepass, the adsorption system can readily maintain a maximum tolerableacetylene concentration of two parts per million in the liquidsurrounding the tubes of reboiler 4. On the basis of completely removingthe acetylene from the oxygen-rich liquid in one pass through theadsorber, a circulation rate equal to the total rate of vaporizing theoxygen-rich liquid is sufficient to prevent the accumulation ofacetylene. However, as it is always advisable in commercialinstallations to provide for certain inefficiencies of adsorption suchas may be encountered when a bed of adsorptive material has been onstream for some time and is approaching need for regeneration, thisdescription is presented from the point of view that the adsorbereffectively operates at efficiency. Thus oxygen-rich liquid whichcontacts the adsorber with two parts per million of acetylene and leavesthis contact with one part per million necessarily must have acirculation rate approximately double the minimum circulation rate ofcomplete adsorption, or equal to the feed rate in mols per hour of theatmospheric air to the process. A circulatory flow through theadsorption system approximating at least a circulation rate equal to thefeed air rate to the separation process eliminates the phenomenon oftotal vaporization heretofore employed in the bottom of rectificationstage 3 and thus eliminates the certainty of acetylene precipitation atthis point. Furthermore, even if the feed air were to have an acetyleneconcentration of two parts per million increasing the circulation rateof liquid through the adsorber permits the adsorbent to reduce theacetylene concentration in the pool of liquid around the tubes ofreboiler 4 to below one and a half parts per million and thus definitelyprevents formation of solid particles of acetylene.

Figure 1 illustrates a preferable method vfor circulating theoxygen-rich liquid by thermal flow. By this method, reboiler 4 isoperated to vaporize 17,000 pounds of liquid per hour. Meanwhile, 57,200pounds per hour of liquid is withdrawn through line 5 and taken throughone or the other of connecting lines 0 or l, having valves 8 and 9respectively, for introduction into either of the adsorbers I0 or II.These vessels contain granular adsorbent material Which preferably issilica gel. The beds may be of any suitable arrangement but convenientlythe silica gel is placed in an annular space between two perforatedcylindrical walls to form the bodies of adsorbent denoted by thenumerals I2 and I3 respectively. The beds of adsorptive material I2 andI3 are necessarily designed to accomplish the desired degree ofadsorption and in the present illustration, have a volumetric capacityof 18.7 cubic feet. This capacity is Sullicient to retain approximately748 pounds of silica gel in a mass of such density as to cause onlyabout 0.2 pound per square inch pressure drop through the adsorber whichrepresents a design based on twice the required circulation rate and 41%of the maximum allowable space velocity through the beds. The incomingliquid enters the inner space of beds I2 and I3 and then passes more orless horizontally therethrough into an outer space confined by the wallsof adsorbers egebogesa .lfand I.I. FInspassingmthrough the adsorbent ,at.the .'flowfconditions of this operation, the ,oxygen-rich liquid issubstantially completely denudedof its acetylene content by the vtime itis .withdrawn from ,the adsorbers through either .offtheconnecting linesI4 or i5, havingvalves 1:6 ,and A|'| respectively. It is possible thatthe actionbyfbeds I2 and -|'3 is not effected by adsorption ybut `byabsorption orby both actions. Whenever the term adsorption is usedhereinafter, `itconsequently also includes absorption .-.Atpcriodicintervals, which may conveniently be,.once=everye24 hours, itbecomes necessary to .regenerate the adsorbers. This is doneconveniently by-.closing valves l8 and it or 3 and VEl?, ,depending.upon -which ofthe adsorbers iii or is'undergoing regeneration, openingthe corresponding pair of .valves for the other adsorbervandopening-valve |-8rin line i9. When valve |53 isopened, nitrogenvapor is taken from line 20 and. passed throughline i9 to heater 2Atthis pointthe temperatureof the nitrogen is raised y ltoa.temperature.suitable for purging acetylene .from the adsorbent material, lThelheated nitro- .gen yvaponis then taken from` the heater through.line-22 and passed by Way of either or" the connecting lines 23 or .24,valve 2.5 or 25 respectively .being.open,.into.1ine.6 or l for passagethrough. .the adsorber undergoing regeneration. The regenerating vaporcommingled with vaporized acetyleneleaveseither-adsorber it or li by way.of line yHl or |.5 .andis .purged from the system byr.either,purgeline..2.| or=28, valve 2.9 or 35E being .open for .this .purpose.

After .undergoing contact with the silicagel .in the adsorption .,step,the acetylene-denuded liquidis passed to the .outside reboiler 3| by wayof line .32. For the case in which the product is Withdrawn 'from therectification in the .liquid phase, it may be. conveniently removed fromline 3.2'by line 44, valve 45 being opened for this purpose. In anyevent, it is thefunction of out- .side reboiler 3| to takeacetylene-denuded liquid `passing theretoin line 32 and to begin andmaintain a positive circulatory flow by a so-called thermal siphonaction created by vaporizing that portion of the liquid which must bevaporized over and above the vapors evolved by reboiler 4 so as to makethe total reboiled vapors, or withdrawn liquid product and reboiledvapors, equivalent to the liquid downiiow. In the present operation witha vaporized product, these vapors amount to 13,750 pounds per hour and,forsupplying the necessary heat to produce these vapors. .valve 33 isactuated to bring 17,351 pounds per hour of vaporous nitrogen from thetop of the high pressure rectification stage through line 20 Vtoreboiler 3| at a temperature of approximately 280 F. These Warm vaporspass through the shell side of reboiler 3| on the outside of tubes 34,in countercurrent heat exchange relation with the acetylene-denudedliquid passing upwardly through the tubes, and are condensed thereby.Nitrogen condensate leaving the reboiler at a temperature ofapproximately 281 F. through line 35 ,may .be combined with additionalamounts Withdrawn from tray 35 through line 3l if such is necessary andthen passed to an upper point in rectification stage 3 in a manner notshown on the drawing. In the event it becomes necessary to employ alarger quantity of nitrogen vapors for the above heat transfer heatrelationship, the excess condensate is returned to high pressurerectification stage 2 by way of line 38, having check valve 39positioned therein to vprevent backward-return ofany liquid from recti-.tion .between .the liquid .and vapor phases.

iication:v stage .-2. :The-vaporovus fandrunvaporized portions iof theHacetylene-denuded liquid :leave reboiler .3| at about 290 F. through.line '.40 and Vare :conducted: through this :line backftothe low.pressure rectification vsta-ge 3. Should .it -become necessaryordesirable .to changethecirculation rate throu'ghithe -adsorptionsystem, ,this is done by employing Vvalve 33 asacontrol valve. Forinstance, thevalve may be openedesomewhat .to divert-more of thecondensing nitrogen ,to outside reboiler Y3|. .Thefresultantsefect4of-this diversion .would bef-to increase-the vaporization ratein thereboiler 'therebyrestablishing Aan increase of liquid .iiow through they.circulatory adsorption .system and correspondingly .decreasing the-vaporizing n.eifectof .inside reboiler 24.

.In'the present illustration, to ;achieve.a..suitable thermal .-ilow Vofliquidthrough .thek circulatory adsorptionsystemflines- 5 .and 32 andtheconnecting lines vii, l, lr-and .I5 have vbeen-designed so as to createnomore than'about 0.33 pound per .squareinchpressurer drop` tor theinletof reboiler 3| plus, .ofcourse-the 0.2 pound,per square inchpressure-drop througheitheiuof ,the adsorbers. The pressuredrop through.theputside. reboiler ..3 .has 4beendetermined to vbe .of .theorder.of.0.24 pound per square inch whileline te is constructed ofsuitable.cross-sectional..area to have a pressuredropof about..22..po.und .per square inch. The summation of .pressure drops throughyeach part .of the system. determines the required. drivingforceforactuating. the. speciiied circulation rate of the oxygen-rich.liquid .therethrough. 'I'his .driving force is created .by .the

vdensity diierencebetweentheffluids in reboiler ,3i and the bottomofrectification stage "3 as Well as by the positioning of Voutsidereboiler 3| ata level below theaverage level .of the .liquid1poolsurrounding the tubes of .reboiler '.4 while .also taking into.consideration the ,pressure .'liead necessary .for Vreturning condensednitrogen ,to thetop tray of rectiiication stage 2.

The materialfromline VAllis returnedtorecti- Acation stage 3 in thevapor space above .the

surface ofthe liquid.aroundreboiler-A andbe- 10W the bottom rectifyingtray to effecta separa- `In this illustrative ,procedure, 13,750 ,poundslper hour of vapors .are derived by -thejheating .ob- .tained in outsideAreboiler 13| .and .these vapors, combined with the 17,000 .pounds .perhourpof vapors derivedfrom. the' heatingV effect. of reboilery1|,.provided both the ,11,165-pounds .per hour .of

oxygen-.rich product, Withdrawnthroughline 46, and .19,585 pounds perhour vaporous upilowior .the rectiiication. Simultaneously, `43,450v.pounds per hour .of acetylene-.free liquid likewise are .derivedfrom.line 40 and this liquid .once ,again becomes commingled with the.oxygen-rich ,liquid surrounding the tubes of .reboiler.4, and by .thiscommingling the acetylene Aconcentration in the liquid pool at the baseof rectiiication stage 3 is maintained at approximately one part .permillion.

Alternatively, tothe process 4iovv arrangement described hereinabove,reboiler 4 maybe -omitted .and its functionincluded in Htheperformanceof outside reboiler 3|. Reboiler 'mayberemoved .from operation-byany-meanssuch as iilling'fthe tubes With inert gas or convenientlybyblocking off the tubes thereof to the flow of nitrogen vapors, as by amovableperforatedcover plated This .plate Ais retained in position fbysuitable vbracketsin a manner to permit a desired free- .dom ofmovement. The platecontainsperforations comparable in location and sizeto the location and inside diameter of the tubes of reboiler 4. A rod 42is attached to plate 4| and extended through the wall of therectification column being properly fbushed to prevent leakage aroundthe rod. By means of a hand device 43 outside the column, rod 42 can bemanipulated so as to change the position of plate 4| in its retainingbrackets enough to entirely block the upward flow of vapors into thetubes of reboiler 4. In this event, the pool of oxygen-rich liquid atthe bottom of rectification stage 3 becomes a reservoir to receive andhold both the liquid downflow of rectification and unvaporized portionsof the acetylene denuded liquid from line 40. The oxygen-rich liquid forcirculation through the adsorbingand vaporizing steps is then withdrawnthrough line from this reservoir. Circulation of the oxygen-rich liquidfrom and to the bottom of rectification stage 3, as before, must be atsuch rate as necessary to maintain the acetylene concentration thereinbelow the main concentration required for safe operation. With omissionof reboiler 4 all the vapors of nitrogen passing to the top ofrectification stage 2 are taken through line 2|] for condensationthrough outside reboiler 3|. The total liquid reflux for the highpressure rectification step isthen returned from reboiler 3| by way ofline 38.

`While circulation of the oxygen-rich liquid through the circulatoryadsorption system has been described as being sustained by the thermalflow conditions provided by the outside reboiler 3|, the same iiowconditions may be maintained by interchanging the functions of theinside and outside reboilers. Figure 2 diagrammatically represents aprocess arrangement wherein the positive flow of liquid through theadsorption system is provided by a reboiler located within rectificationcolumn According to this processing method, the design of the insidereboiler is changed somewhat, for example by a heat exchanger 50 forreboiling which is positioned intermediately between rectificationstages 2 and 3 but entirely separated therefrom by a bottom plate 5| anda top plate 52. All of the nitrogen vapors are now withdrawn from thetop of recti` flcation stage` 2 through line 20 butpart of these vaporsare diverted from line 20 into line 53, introduced therethrough into theshell side of the tubes 54 of reboiler 50, and are condensed by heatexchange relation with oxygen-rich liquid Vpassing upwardly within thetubes. The condensed nitrogen is then withdrawn fromreboiler 50 andpassed to the top tray 36 of rectification stage 2 by way of line 55having check valve 56. The other part of the nitrogen vapors passingthrough line 20 are introduced into the shell side of the tubes 34 ofoutside reboiler 3|. The quantity of vapors fiowing through line 53, iscontrolled by the setting of control valve 33. Within reboiler 3| thenitrogen vapors pass downwardly in countercurrent heat exchange withoxygenrich liquid rising through the inside of tubes 34 and arecondensed thereby after which the condensate is removed through line 35as heretofore described. Any of the nitrogen condensate needed forrefluxing purposes in rectification stage 2 is taken from line 35through line 38 to the upper tray 36. Check valve 39 prevents back-flowof liquid through line 33 in the event of pressure fiuctuations inrectification stage 2.

i Referring now specifically to rectification stage `3.of Figure 2,oxygen-rich liquid downowing from the vapor-liquid contacted in thisstage, is collected on tray 51. This liquid is removed from the traythrough line 58. Simultaneously, an out-flowing mixture of vapor andliquid from the top of reboiler 50 is taken through line 59, havingcheck valve 60, and passed into line 58. Thecommingled streams in thisline are introduced into the bottom of reboiler 3| andV brought intoheat exchange relation with condensing nitrogen vapors from line 20 forfurther vaporization. The vapors are removed from'the top of reboiler 3|and returned through line 40 to the vapor space below the lower mosttray of rectification stage 3. Product vapors are withdrawn from thisspace through the valved line 6| and the vapors not so withdrawn passupwardly through the several rectifying trays of the stage. The productvapors alternatively may be` withdrawn directly from line 40 through thevalved line 62. That part ofthe oxygen-rich liquid not vaporized inreboiler 3| is removed from the base of the reboiler through line 63 tothe adsorption stage. There, the liquid undergoes adsorption in themanner as heretofore described in connection with Figure l after whichit is subsequently caused to flow thorugh line 64 into the bottomsection of reboiler 50 to undergo the necessary heatingfor promoting thepositive flow through the adsorption circuit. When a liquefied productis desired, this product may be withdrawn from line 64 by valved line 66and conditions adjusted in the operation of the liquid recirculation topermit this withdrawal. In case there is an accumulation of liquid inthe bottom of rectification stage 3, such liquid may be drained from thestage through valved line 65 and added to the liquid flowing throughline 64.

Figure 3 illustrates the application of the method of the invention in arectification zone wherein all of the reboiling for thelow pressurestage is effected by inside reboiler 4. In this figure only theessential parts of the apparatus that is necessary for the descriptionhas been shown as it is understood that rectification tower I separatesair according to the conventional two stage operation for suchrectification. Likewise, only one absorber vessel is shown, it beingunderstood that this vessel may be taken off stream for regeneration anda second one used for adsorption in the manner heretofore described. Themethod of purifying the oxygen-rich liquid follows the ,procedure asdescribed in connection with the Aforegoing figures. Line |00, having anexpansion valve |ll|, connects the bottom of high pressure rectificationstage 2 with an intermediate point of low pressure rectification stage 3for the purpose of transferring liquefied oxygen-enriched `air from theformer to the latter-mentioned rectification stage. In the presentinstance, however, 57,200 pounds per hour of the oxygen-rich liquid `arerecirculated from rectification stage 3 through line 5 under positiveflow conditions maintained by pump 1G to adsorber I0. Upon its passagethrough the bed of adsorbent in absorber I0 the oxygen-rich liquid issubstantially denuded of its acetylene constituent when it is passedthrough line 1| back to rectification stage 3. The continuouscommingling of the liquid from line 'Il with the body of liquidsurrounding the heat exchange tubes of reboiler 4 maintains theacetylene concentration of the liquid body at approximately one partper` million. The present procedure is particularly adaptable forseparations producing substantially pure liquid oxygen as such productmay be removed conveniently from line 1l by the valved line 12. Avaporous product may be recovered alternatively directly from therectication tower through valved line 13.

While it is a convenient practice to Super-im- .pose rectication stage 3above stage 2 in a single integral vessel, such an arrangement is not anecessary requisite for the rectification. The modication of thecirculatory acetylene adsorption system illustrated by Figure 4 isparticularly suitable in its application to rectication stages whichperform their function in separate vessels positioned side by side. Inthis case positive circulation of the oxygen-rich liquid between thebottom of low pressure rectication stage 3 and reboiler 14 is maintainedby a pump 18 by way of lines 19 and 80 and the intermediatelydisposeduadsorber l0. By employing a pump to maintain positive owconditions, the function of reboiling for stage 3 may be carried outentirely by condensing nitrogen vapors in reboiler 14 at the top of thehigh pressure rectification stage 2. This permits the convenient returnof reflux liquid to stage 2 by gravity flow through line15. Condensationof the nitrogen vapors outside the tubes in reboiler 14V partiallyvaporizes the oxygen-rich liquid passing upwardly inside the tubesvafter delivery thereto from the low pressure recti'cation stage 3. Themixture of vapors and liquid thus formedv in reboiler i4 are passedthrough line 16 into the vapor space 11 below the bottom tray of stage 3which space serves in the dual capacity oi separator and liquidreservoir. From'this reservoir the oxygen-rich liquid is pumped by pump18 by way of' line 19 through adsorber vIll and then line 86 back intothe botltom section of reboiler 1li.v Lines 91 and 98 connect betweentower 2 and tower 3 for transferring oxygen-enriched liqueed air andliquefied nitrogen respectively from the higher pressure to the lowpressure rectication stage. Expansion valves 102 and |03 are positionedin lines 91 and 98 respectively to exp-and the material beingtransferred therethrough from the higher pressure of rectification stage2 to the lower pressure of rectication stage 3. An oxygen-rich liquidproduct may be removed from line 89 through the valved line 8l or in theevent a vaporous vproduct is desired such product may be removeddirectly from the low pressure rectification stage 3 through valved line82.

The aforedescribed applications of the invention has been illustrated byFigures 1 and 2 with lreference to two stage rectification for aseparation. It is to be understood that the invention is equally asapplicable to single stage rectification which may be more clearlyunderstood by reference to the diagrammatic process flow arrangementsillustrated in Figures and 6. For example, vby now referring to Figure5, compressed and partially liquefied air is introduced by Way of line33 into reboiler S4 wherein it relinquishes its heat of condensation andis totally in the liquid phase as it ilows through line 85 and isexpanded through expansion valve 85 into `the top of rectiiication tower81 as reflux liquid. In tower 81 the liqueed air is rectied into avaporous nitrogen-rich top product which is withdrawn from the towerthrough line 88 and an oxygen-rich liquid bottom product whichaccuinulates inthe base of the tower. The heat liberated in reboiler 84by the nal condensation of the partially liquefied feed air from line 83creates a thermal Siphon effect and by thermal flow causes thecirculation of the oxygenrich liquid from the base of tower 81 throughline 89, the'bdy cf adsorbent in. adsorber 9o and line 9| into reboiler84. In this vessel the` liquid is partially vaporized by the heatexchange with the condensing air so that it is a mixture ofacetylene-denuded vapors and liquid which are returned to tower 81through line 92. Within the tower the vapors are separated from thevliquid and a portion of them taken `as product by way of line Q3, whilethe remainder pass upwardly through the rectifying steps of the towei`invap-or-liquid contact with the downlowing reflux liquid. Theunvaporized material from line 92 commingles with the oxygen-richliquidi at the bottom of tower 81 and subsequently is recirculatedthrough the adsorption circuit. When a liqueed oxygen-rich product` isdesired the bottom product of the rectication may be removed by way ofline 9d instead of line 93 and the operating conditions of the airseparation plant re,- adjusted to'supply the air to inlet line 83 at theproper temperature to maintain the described flow procedure.

The process arrangement diagrammatically shown by Figure 6 isessentially the same as that liust described for Figure 5. It differstherefrom in that a portion of the partially liqueiied feed air torectifying tower 81 is totally liquefied ina b ottom section of thetower by heat exchange relation with the liquid bottom product ofrectification in reboiler 95. The liquid air thus obtained is thereaftertaken through line 96 and combined with the liquid air passing throughline Vfrom reboiler t4. Optionally, the liquefied air inline 3bv may beseparately introduced into the topof tower 81 through a separateexpansion valve. In this arrangement, part of the vapors from theoxygen-rich liquid at the base of the tower are produced by an insidereboiler and the remainder in the. outside reboiler 84 which functionsin the manner as described for outside reboiler 3| of Figure l.

The oxygen-rich product again maybe withdrawn either vin thevliquidphase through line `9-I or in the vapor phase through line 93,the conditions of operation being suitably adjusted for producing eithertype of product.

Having thus described my invention, whatI claim and desire to secure byLetters Patent is:

l, In the fractionation of air, containing acetylene as anv impurity, byliquefaction and'rectiiication at relatively low temperatures, whereinoxygen-rich liquid is continuously added toa pool in the 'bottom of arectication zone, and V.continuously"kreboile'd from said pool, a methodfor preventing explosions which comprises: continually circulating a.stream of liquidcontaining dissolved'acetylene from saidfpool through abody of acetylene adsorbent material to remove dissolved. acetylene, andback to said'pool at a rate at least equivalent to the amount of liquidva- `porized by the reboiling of-said pool, and suiiicient to maintainthe acetylene in said pool at less than saturation, thereby preventingthe existence of acetylene crystals in Contact with said pool.

2. rIn the fractionation of air, containing'acetylene as an impurity,yby liquefaction and recti- -cation in two stages at low temperatures,wherein said air is fractionated in afirst stage under Substantialpressure into a substantially pure nitrogen vapor fraction and a liquidfraction of increased oxygen and acetylene concentration, and whereinsaid liquid'fraction is fractionated in a second stage under lowerpressure into vapor fractions whicliare rectified in a vrectificationzone above a reboilingpool ofliquid oxygen containing -dissolvedacetylene in still higher concentrations than in said first stage, themethod of preventing explosions which comprises: continually circulatinga stream of liquid oxygen, containing dissolved acetylene, from saidreboiling pool through a body of acetylene adsorbent material and backto said pool at a rate of liquid flow not less than the `rate ofvaporization from said reboiling pool and sufficient to maintain theacetylene concentration within said reboiling pool below saturation,thereby preventing the existence of acetylene crystals in contact withsaid pool.

3. The method in accordance with claim 1 in which the acetyleneadsorbent material comprises silica gel.

4. In the fractionation of air containing acetylene as an impurity,wherein compressed air is rectified in a first rectification stage intoan oxygen-enriched liquid fraction and a substantially pure nitrogenfraction, wherein the fractions are again rectified together under lowerpressure in a second rectification stage and a liquid predominantlyoxygen rectification product containing dissolved acetylene is passedinto a liquid body thereof in a lower part of the second rectificationstage for reboiling by heat supplied by condensation of thesubstantially pure nitrogen fraction of the first rectification stage;the improvement which comprises continuously withdrawing portions ofsaid liquid body, passing the withdrawn portions by gravity flow throughsilica gel to a second body of the liquid predominantly oxygenrectification product externally positioned with respect to saidrectification stages thereby adsorbing dissolved acetylene from thewithdrawn portions, reboiling and partially Vaporizing said secondliquid body by condensing at least part of the substantially purenitrogen fraction to produce an outfiowing overhead mixture of vaporsand liquid from said second liquid body sufficient to establish andmantain the gravity flow of the withdrawn portions through the silicagel, and continuously returning unvaporized portions of the outfiowingmixture from said reboiled second liquid body to the first mentionedliquid body of the liquid predominantly oxygen rectification product andthereby preventing precipitation of solid particles of acetylene fromthe liquid body of predominantly rectification product during thereboiling thereof.

5. In the fractionation of air, containing a-cetylene as an impurity,wherein compressed air is rectified to produce an oxygen-enriched airfraction and a substantially pure nitrogen fraction, wherein thefractions are again rectified together under lower pressure with thecollection and reboiling of a liquid body of oxygen-rich liquidcontaining dissolved acetylene and the heat for reboiling is supplied'by condensing vapors of the substantially pure nitrogen fraction; theimprovement which comprises continuously passing a stream of oxygen-richliquid from the liquid body through acetylene-adsorbent material at arate not less than the rate of collection of oxygenrich liquid in saidbody whereby the adsorbent removes dissolved acetylene from the streamat least at the same rate at which acetylene is entering the liquidbody, then separately heating the purified oxygen-rich liquid of saidstream by a separate heat exchange with a controlled amount of vapors ofthe substantially pure nitrogen fraction and vaporizing enough of theliquid of said stream to effect by thermal syphon ow passage `of thestream of oxygen-'rich liquid through the adsorbent material at saidrate not less than the rate of collection, and thereafter returning tosaid liquid body of oxygen-rich liquid being reboiled an unvaporizedportion of the oxygen-rich liquid from said separate heat exchange in anamount and at an acetylene concentration sufficient to maintain theacetylene concentration in the liquid body below the acetylenesaturation point thereby preventing precipitation of solid particles ofacetylene from said liquid body.

6. In the fractionation of air, containing acetylene as an impurity.wherein compressed air is rectified to produce an oxygen-enriched airfraction and a substantially pure nitrogen fraction, wherein thefractions are again rectified together in a rectification zone underlower pressure with the collection of an oxygen-rich liquid containingdissolved acetylene; the improvement which -comprisese maintaining abody of the oxygenrich liquid in said lower pressure rectification Zone,continuously passing a stream of oxygenrich liquid, containing dissolvedacetylene, from the liquid body at a rate not less than the rate ofcollection thereof through acetylene-adsorbent material to adsorbdissolved acetylene, passing the purified stream of oxygen-rich liquidto a second liquid body, heating the second liquid body of oxygen-richliquid by heat exchange with vapors of the substantially pure nitrogenfraction in a heat exchange zone externally positioned with respect tosaid lower pressure rectification zone, vaporizing at least enough ofthe liquid of the second liquid body to carry a mixture of vapors andliquid overhead therefrom sufficient to effect movement and passage bythermal syphon flow of said stream of oxygenrich liquid through theadsorbent material at said rate not less than the rate of collection,thereafter separating from said mixture an unvaporized portion ofoxygen-rich liquid and passing it to said first-mentioned liquid body ofoxygen-rich liquid in an amount and at an acetylene concentrationsufcient to maintain the acetylene concentration in the first-mentionedliquid body below the acetylene saturation point thereby preventingprecipitation of solid particles gf dacetylene from said first-mentionedliquid '7. In the fractionation of air, containing acetylene as animpurity, wherein compressed air is rectified to produce anoxygen-enriched air fraction and a substantially pure nitrogen fraction,and wherein the fractions are again rectified together under lowerpressure with the collection of a liquid 'body of oxygen-rich liquidcontain-l mg dissolved acetylene which body is reboiled by heat suppliedfrom condensing vapors of the substantially pure nitrogen fraction toproduce a vaporous oxygen-rich rectification product and vapors for thelast-mentioned rectification; the improvement which -comprisescontinuously passing oxygen-rich liquid from said body through anacetylene-selective adsorbent material at a rate not less than the rateof said collection of oxygen-rich liquid from the second-mentionedrectification and sufficient to adsorb at least two parts of acetyleneper million parts of oxygenrich liquid passed, then separately heatingthe passed oxygen-rich liquid by a separate heat exchange with acontrolled amount of vapors o1' the substantially pure nitrogen fractionand vaporizing enough of the last-mentioned reboiling assai-Be liquidtoproduce== an outowingf mix-ture of Yvai-- porsiV and liquid?i andzeffect' by`- athermal syphon iovf'thepassage I ofV oxygen-rich liquidfrom said b'o dy and lthrough''-thefadsorbentmaterialat said rate;separati-ng liquidi portionY of4 oxygen-rich liquidi:- fionrthe;Voutiowing mixture, and:` intro'- dicingAL trie-liquid portion into'saidlbodyfinamounti andy at'- anacetylene concentration'- sui-V 'cient' tomaintainIl the acetylene concentration in-saidf body' below-1 aboutthree parts V4of-"acetylene per' miliionl parts off' oxygen-richfliquid" thereby preventing precipitation vof solid particles-ofacetylene from-said-liquidbody-during-the-reboiling--thereorf-.

In `thefractionation. of air-",ccntaining-V acete ylene -a's animpurity;whereinairfisrectiiied into nitrogen-richandli oxygen-richA vaporousIoutput products-byreboilingivapor'sifroml af body ofoxygeni-ricliliquid, .ini av lowerip'ortion.- ofi thefrectification VLz'on'ei; themethod hof; preventing.l the pre' cipitation': of acetylene` crystalsfrom saidi. liquid bod-yf which comprises continuously` removing astream-:off oxy-e'en-richlliquid'.containing-.dissolved acetylene fromsaidlbody, continuously passi-ng the; removedi streaml through; anacetylene -selectire.-k adsorbentff materiali at a'. ratei not.- lessthan the':rate-ofz'vaporization from said oxygen-rich liquidbodyi tocontinually` adsorbfdissolved acetyleneI fromsthe stream, heating thethus puri-- fied; oxygen-rich? liquider. said streamby heatexc-hangelfwith an' innowing stream ofi partially condensed'iair?under.- higher pressure passing to said rectiiicationn zone-:inaheat exchange zone externally.- positionedwithirespect to said"rootincation-mono;r vaporizing enough` of the purined liquid.yto'produce:` an outowing mixture of` vapers: and liquid r suicienttolsupply the vaporous oxygenerich-v output product and: vapors for' saidrectification and'to: effectby a thermal syphon flow the: passage of`the stream withdrawn from theA liquidsbody through the.adsorbentmaterial at said: rate;,. separating-1 ai liquid portionl of'oxygen-rich liquid fromthe. outowing mixture and inti-oducimg` theliquid. portion.- into said' bo'dyin anaamount-and'y at an acetyleneconcentration sufficient to maintain the acetylene concentrationfin-4theliquidbodyy below theacetylene saturationpointt-herein.` l 9;. Infthe l fractionation ofV air,Y containing acetylene as an impurity,wherein partially condensed compressed air is rectiiiedl in'. a-=iirst-rectication stage intoA a; liquidf oxygen-enriched fraction' and asubstantially pure nitrogenf fraction; wherein thefrac-tionsareaganrectiied.- together under lowerv pressure: ina secondirectification stage withthe. collection and,v reboilingof? ani oxygenrich:liquidcontainingldissolved acetylenes by heatsupplied-from-condensing vapors of the substantially pure-nitrogen"fraction' inthe rstllrecti cationistage. to: produce a Vaporousoxygen-rich rectificationproduct and vapors for the secondstagerectication; theY improvementwhich' com'- prises withdrawingfromthe liquidi;v bodycollected under lower pressure a stream of" oxygenrichliquid-v containing dissolved acetylene;l passing'i'thie withdrawnstreainivunder thermal syphon flow conditions ata rate not-less thanthe'rate ofivap'orization' from said liquid body throughanacetylene-selective" adsorbent material. to. adsorb dissolved-'-acetylene from' said stream, thenpassing the thuspuried oxygen-richliquid. of. said streamfin-heat-exchange withV a separate stream ofpartially-f condensed compressed air passing to thesecondffrectiiication stage in a heat" exchange zone externallypositioned| withlresp'ect to" said second" rectification stage,-thereafter" combiningtheseparate stream off partially condensed com'-pressedV air' withastream of the liquidoxygenenriched? fraction Vof theAii'rst rectiiicationV stage and.. ezrpanding tl''eV combined'streamsinto said Secondv rectication stage', vaporizing4 eno'ughof the puriiedoxygen-rich liquid in the heat exchange zone to'prodiice an outflowin'gfmixture of! vapors and liquid to establish and maintain thev ther-mali`syphorr flowV ofthe withdrawn4 stream ati saidfr'ate, separating liquid*portion of oxygenrich liquid from theJ outflowingf mixture and' invtroducing the liquid portion into the liquid body of oxygen-rich liquidbeing reboile'dinthelsecond rectiiic'ation stage in-an amount and atanacetylene con-centration suicient to maintain the acetylene?lconcentration in the liquid body below the' acetylene saturation pointtherein-andftliere-f byl preventing precipitation of solid particles' ofacetylene from saidliquid body duringr the re'- boilingthere'of. Y

10. f In thefractionation of air, containing acetylene as an impurity,wherein partially con'- densedcompiessedair is rectied'in afirstrectiiication'V stage into-A a liquid oxygen-enriched fractionand'laV substantially pure nitrogen-frac tion, whereinthe fractions areagain rectifiedtqgether under lower pressure in-asecondrectification-stage with the'collection of a body of oxy= gen-richliquid, containing dissolvedY acetylene in thesecon'd stageArectification; therimprove'- ment whichk comprises withdrawing from theliquidbodyin the se-cond rectification-stage a'stream orv thevoxygen-rich liquid containing dissolved acetylenazpassing-'the withdrawnstream through an acetyleneeselective adsorbent material at` a ratenotless than the rate-of said'collection' of oxygen-rich liquid in theliquid body toadsorb acetylene-from' thestream at least atl the samerate'y at' which acetylene is entering theV liquid body, removing" thepurified oxygen-rich liquid strearn` from contact-'with theadsorbentmaterial. sepa-rating fromv the-purified'stream a liquidoxygen-rich--outputproduct of the air fractionation, passing. theremainder of thepuried stream. in heat exchange.relationwith vaporsofthe substantially' pure'nitrogen-fraction of the firstIrectnt-icatonstage' to' produce at least sufficienti vapors.forthe'second stage rectification, separating anunva-poriz'ed portionof the puriedoxygenrich liquid after the last-mentioned heat" exchange:and'` introducing 'said unvaporized portion into-the. body-ofoxygen-rich liquidl in anamount and at an acetylene concentrationsufiicient to maintain the-acetylene concentration in` the liq.- uldbody belowtl'iey acetylene saturation point therein and therebypreventing precipitation of sol-1d particlesofxacetylene-from saidliquid body-` WALTER E. LOBO.V

References Cited in the lle ofv this patent' UNITED-f STATES PATENTSNumberr-

1. IN THE FRACTIONATION OF AIR, CONTAINING ACETYLENE AS AN IMPURITY, BYLIQUEFACTION AND RECTIFICATION AT RELATIVELY LOW TEMPERATURES, WHEREINOXYGEN-RICH LIQUID IS CONTINUOUSLY ADDED TO A POOL IN THE BOTTOM OF ARECTIFICATION ZONE, AND CONTINUOUSLY REBOILED FROM SAID POOL, A METHODFOR PREVENTING EXPLOSIONS WHICH COMPRISES: CONTINUALLY CIRCULATING ASTREAM OF LIQUID CONTAINING DISSOLVED ACETYLENE FROM SAID POOL THROUGH ABODY OF ACETYLENE ADSORBENT MATERIAL TO REMOVE DISSOLVED ACETYLENE, ANDBACK TO SAID POOL AT A RATE AT LEAST EQUIVALENT TO THE AMOUNT OF LIQUIDVAPORIZED BY THE REBOILING OF SAID POOL, AND SUFFICIENT TO MAINTAIN THEACETYLENE IN SAID POOL AT LESS THAN SATURATION, THEREBY PREVENTING THEEXISTENCE OF ACETYLENE CRYSTALS IN CONTACT WITH SAID POOL.